An example of such a plug cap is described in JP-A-6/196246 (1994). The plug cap is traversed by an ignition cable and is freely mountable and removable with respect to an elongated bore which houses a spark plug. When fitted into the bore, the plug cap serves to prevent the ingress of water and, at the same time, holds an ignition pipe (also called "plug cap pipe") firmly in place.
When the engine is running, the air inside the elongated bore is heated and tends to expand, causing the plug cap to lift away from and float above the elongated bore. To avoid this, the plug cap is provided with one or more vent-holes at its upper face or side face to release the expanded air.
However, when the vent-holes extend upwards or sideways, they are vulnerable to water penetration and thus impair the sealing properties of the cap. Thus, when water is sprayed at pressure on such a plug cap, for instance when driving in the rain or washing the car, there is always a risk that water will penetrate the elongated spark plug bore. When this happens, electrical leakage may occur from the spark plug through the water. The plug can thereby fail to ignite properly, with adverse effects on the running of the engine.